126 RAYMOND PEARL 



an older terminology, nor does it essentially contribute to our 

 knowledge about them. Indeed it is, so far as I can see, entirely 

 fair to say that but little in the way of essential advance has 

 been made towards the solution of this problem since Darwin's 

 examination and analysis of it. The two leading students of 

 variation since Darwin (Bateson and De Vries) have, to be sure, 

 contributed greatly to our knowledge of certain aspects of the 

 phenomena of variation; notably, on the one hand, in the direc- 

 tion of estabhshing a number of definite principles or laws of 

 morphogenesis which control or determine in large degree the 

 somatic expression of germinal differences, and, on the other 

 hand, in very precisely and minutety analyzing the genetic be- 

 havior of various heritable variations, after they have appeared. 

 But it is the problem of the origin, the determination, the causes 

 of those germinal differences which lie behind somatic varia- 

 tions, and indeed are the heritable variations, which appears to 

 be the basic problem of genetics (Pearl, 21, p. 39). 



One possible method of attacking the problem of the origin 

 of heritable variations is that suggested by the quotation which 

 stands at the beginning of this paper. One may systemati- 

 cally expose the germ-cells of an animal to something unusual 

 or abnormal ''in the surrounding conditions," and then analyze, 

 so far as may be, not only the new heritable variations them- 

 selves (provided any such appear), but also the factors which 

 underhe their causation. One is the more encouraged to under- 

 take experimentation in this direction, because of the very in- 

 teresting results of such studies which have been reported dur- 

 ing the last few years, particularly those of Stockard (34, 35, 

 36, 37, 38), and Cole and his students (3,4), with mammals and 

 birds. In this connection mention should also be made of the 

 work of Sumner (39) with mice, Kammerer (13) with lower 

 vertebrates, Tower (41) with insects, and MacDougal (15) with 

 plants. No attempt will be made at this point to review this 

 literature. Such observations of these others workers as re- 

 late to the results here reported will be discussed later in this 

 series of papers. 



In discussing with Professor Stockard in 1914 his very inter- 

 esting results on the effect of chronic parental alcoholism upon 



